I had the same problem, wanted a mandarin, no refugium, so, I made one!
My tank is a room divider, so it can be seen from both sides, so my bak-pak protein skimmer is on the side.
I came up with this idea. I built a wood box on the end of my tank, put the protein skimmer at the back of the box, using pvc to hard line it in. I then took a 2 1/2 gallon betta tank and set it between the protein skimmer and the tank. I filled it with LR rubble and about 2 inches of sand and some calpura macroalge. I made the skimmer empty into the betta tank. I drilled a hole in the other side of the betta tank and used pvc to make it empty into the tank.
Whala! Instant refuge and nursery for copepods!
Ron's homemade cheap refugium!